Process for preparing synthesis resin films of improved stiffness

ABSTRACT

MICROVOIDS FORMED IN AT LEAST REGIONS NEAR THE OUTER SURFACE OF A BASIC SYNTHETIC-RESIN FILM AND COMMUNICATING WITH THE OUTSIDE ARE FILLED WITH A POLYMERIZABLE MONOMER, WHICH IS THEN POLYMERIZED WITHIN THE MICROVOIDS, WHEREBY THE STIFFNESS AND OTHER CHARACTERISTICS OF THE FILM ARE IMPROVED. THE BASIC SYNTHETIC-RESIN FILM IS PREFERABLY A FILM OF A POLYOLEFIN, PREFERABLY A HOMO- OR COPOLYMER OF ETHYLENE, PROPYLENE, OR BUTENE-1, WHILE THE MONOMER IS AN ETYLENICALLY UNSATURATED MONOMER.

United States Patent PROCESS FOR PREPARING SYNTHESIS RESIN FILMS 0FIMPROVED STIFFNESS Yuju'o Nakayama and Mitsuo Yoshiyasu, Yokkaichi,Japan, assignors to Mitsubishi Petrochemical Company Limited andKabu'shiki Kais'ha' Oji Yuka Goseishi Kerlrlkyuio, Tokyo-to, Japan,fractional part interest to eac No Drawing. Filed Aug. 17, 1971, Ser.No. 172,543 Claims priority, appligagigriggpan, Aug. 21, 1970,

Int. Cl. B44d 1/32 US. Cl. 117-7 5 Claims ABSTRACT OF THE DISCLOSUREBACKGROUND This-invention relates generally to synthetic-resin films andmore ,particularlyto the production of new syntheticresin films-ofimproved stiffness.

As one kind of synthetic-resin film, a film having microvoids whichcommunicate with the outside at least in regions near the surface of thefilm can be made. A film of this character can be formed by using afoaming agent and'also 'by stretching aj synthetic-resin film in which afine filler has been blended. Furthermore, a film of this nature may beone having a laminated structure.

A synthetic-resin film of this nature is per se a useful material. Forexample, such a film containing a fine filler blended therein,particularly one having a laminated structure, has high utility as asynthetic paper. (Examples of such films are disclosed in US. PatentAppln.

No. 809,629 filed on Mar. 21, 1969, and now abandoned,

and No. 101,574, filed on Dec. 28, 1970, and now US. Pat. No.3,765,999).

In some cases, however, the stiffness of such syntheticresin films hasnot been entirely satisfactory for reasons such as the kind of rsynthetic resin used. It is apparent, therefore, that if it werepossible to improve the stiffness of these synthetic-resin films, thesefilms would be of even greater value.

SUMMARY It is an object of this invention to realize this improvement ofthe stiffness of these synthetic-resin films. We have found that thisobject and other objects of the invention can be achieved by introducinga synthetic polymer into the above mentioned microvoids of each film.

According to this invention, briefly summarized, there is provided aprocess for producing synthetic-resin films of improved stiffness whichcomprises preparing a basic synthetic-resin film having microvoidsdisposed in at least regions of the film near the outer surface thereofand communicating with the outside, introducing a polymerizable monomerinto these microvoids, and polymerizing this monomer within themicrovoids.

According to this invention in another aspect thereof, there areprovided a synthetic-resin film of improved stiffness comprising a basicsynthetic-resin film having microvoids disposed in at least regions ofthe film near the outer surface thereof and originally communicatingwith the outside and a synthetic polymer contained and polymerizedwithin the microvoids of the film.

3,817,771 Patented June 18, 1974 The nature, utility, and furtherfeatures of this invention will be apparent from the following detaileddescription beginning with a consideration of general aspects andfeatures of the invention and concluding with specific examples ofpractice illustrating preferred embodiments thereof.

DETAILED DESCRIPTION In accordance with this invention as summarizedhereinabove, a synthetic polymer is introduced into the microvoids of asynthetic-resin film by formation in situ thereby to fill the microvoidsand improve the stiffness of the film. We have found that thistechnique, moreover, greatly improves the surfaces characteristics ofthe film. For example, by introducing a polar synthetic polymer such aspolymethyl methacrylate in the microvoids in the case of a relativelyinert polyolefin-resin film, it is possible to improve other propertiesof the film such as adhesiveness and printability.

Furthermore, while synthetic-resin films having a large number ofmicrovoids in regions thereof near their outer surfaces haveinsufiicient surface hardness in many cases, the surface hardness ofsuch films can be greatly improved by introducing a synthetic polymer,particularly a rigid polymer, into these microvoids. A furtheradvantageous feature of this technique is that the surface luster of thestarting-material film also can be improved by using a polymer to fillthe microvoids having high luster such as polymethyl methacrylate.

In the practice of this invention, a synthetic-resin film havingmicrovoids only in regions thereof near its outer surface isparticularly desirable. In this case, since the synthetic polymer isintroduced into only this region near the film surface, it is possibleto avoid such disadvantages as an increase in the specific gravity ofthe film, a lowering of the whiteness or opacity thereof, and anincrease in the processing cost, which accompany instances where thepolymer is introduced even into the interior regions of the film.

This invention can be applied to any synthetic-resin film which has finemicrovoids communicating with the outside in at least regions of thefilm near its outer surface.

Specific examples of synthetic resins of which such films are made are:polyolefin resins such as homopolymers and copolymers of u-olefins,e.g., ethylene, propylene, and butene-l; polystyrene resins such ashomopolymers and copolymers of styrene, nucleus-substituted and (or)side-chain-substituted styrene, e.g., a-methylstyrene, vinyltoluenes,and chlorostyrenes; poly methacrylate ester resins such as homopolymersand copolymers of lower alkylesters; polyvinyl chloride resins;polyvinyl alcohols; polyamide resins, and mixtures thereof. These resinsmay contain fillers and other additives.

The film may be a laminated structure made up of two or more of thesesynthetic resins or of one or more of these resins and other filmmaterials. This film may be in various forms, for example, sheets ofconsiderable transverse width or tapes of relatively narrow width.Furthermore, the thickness of this film is ordinarily of the order offrom 20 to microns.

The microvoids existing in this film in at least regions near the outersurface thereof and communicating with the outside are preferably of adiameter (or the longest dimension in non-circular microvoids) of theorder of from 5 to 20 microns. While microvoids of this character can beformed within the synthetic-resin film by any suitable method, onepreferable method comprises monoaxially or biaxially drawing orstretching (i.e., orientating) a film containing a fine filler blendedtherewith.

That is, preferable films among the synthetic-resin films to which thisinvention can be applied are those produced 3 by stretching singlelayers or laminated film layers of at least one polyolefin resin (e.g.,polypropylene or highdensity polyethylene) containing a fine filler orblended resins of difierent optimum stretching temperatures. Examples ofsuch films are disclosed in US. Patent Appln.

comprises irradiating the monomer with high-energy radiation rays, inwhich case the effect of graft copolymerization may be expected.

Upon completion of the polymerization, necessary after-treatment stepssuch as washing and removing poly- No. 809,629 filed on Mar. 21, 1969entitled Synthetic 5 mer adhering to the film surface are carried out,where- Papers and Method of Making the Same and No. l,- upon a productaccording to this invention is obtained. 574 filed on Dec. 28, 1970entitled Synthetic Paper and Examples Process for Producing the Same.

"In a film of this character, since the microvoids are dis- 10 1. Asheet of a synthetic paper of polypropylene (PP) continuous indirections perpendicular to the film outer which had a surface microvoidlayer and a total thickness surface, the polymerizable monomer does notinfiltrate of 50 microns, was prepared and previously placed in an intothe film interior region. Consequently, the introducevaculated vessel toremove gases from the surface layer. tion of the synthetic polymer islimited to only the sur-- The sheet was then steeped in a mixturesolution of methyl face region, whereby the aforementioned advantagesare methacrylate (MMA) and methanol (MeOH) in a ratio attained. byweight of 7:3.

The polymerizable monomer to be introduced into the After 3 minutes, thesynthetic paper, impregnated with microvoids of the synthetic-resin filmshould be in a MMA solution was taken out of the solution and placedliquid state. That is, it should be a liquid itself, or in the in apolyethylene bag. Samples of this synthetic paper form of a solutionunder the introduction and polymerizawere then irradiated with radiationof 0.5 and 1.0 mrad., tion conditions. In the case where the monomer isin the respectively, at room temperature with the use of Co-60 form of asolution, subsequent removal of the solvent is of 1,500 curies as theradiation source thereby to carry necessary but can be readilyaccomplished since the procout graft copolymerization. es material is afilm. Furthermore, this polymerizable Next, a polypropylene film of amelt index (MI) of monomer should be a monomer which substantially will4 and a thickness of 50 microns was prepared and subnot dissolve thesynthetic resin constituting the film. jeeted to the same Process asdescribed above.

A representative example of a polymerizable mono- The q y ofimpregnating Solution and Percentage mgr f hi character i a compound h iat least one of weight increase with respect to each sample wasdeterethylenically unsaturated bond. Examples of known commilled- Theresults are ShOWn in Table pounds of this character are vinyl compoundsand con- TABLE 1 jugate diene compounds. Such a compound is selected tosuit each purpose. agga; weight Of these compounds, monomers which arecapable of Sa ple dose increase Clark forming polymers of second-ordertransition points above number 83mph (mad) (percent) C. are preferablesince their efiectiveness in increasing 3 10 the stiffness of the filmis excellent. Ethylenically unsatufit 8 rated monomers of this characterare known, specific examples thereof being styrene, methyl methacrylate,acryloa 1 nitrile, and divinylbenzene. 1.0 4

A polymerizable monomer of this specification may 40 be one with which asynthetic polymer is formed by poly- 2. Various films were irradiated bymeans of 2 4-Meb, condensation or polyaddition. A polycondensation reac-750-W, linear accelerator manufactured by the Mitsubishi tion isaccompanied with liberation of a substance such Heavy Industry Co., Ltd.The compiled results are shown as water, alcohols, or ammonia, but sincethe process in Table 2.

TABLE 2 Process conditions Impregna- Absorbed Wt. Sample tion time doseincrease Clark Thickness Specific No. Sample (min.) MeOH (mrad)(percent) stiffness (micron) gravity 10 PE synthetic paper 13 0. 61 080/20 a 0 14 00 0. 02 10 80/20 a 10 20 01 0. 00 10 80/20 8 10 1s 02 0.0020 80/20 3 1s 20 01 0.7 10 70/30 a 14 22 02 0.05 20 70 01 10 70/30 3 20a2 72 0.8 10 70/30 s 18 a0 73 0.8 10 70/30 a a 00 0.90 10 70/30 3 4 000. 00

material is a film, the liberation can be easily carried out. 3. The PPsynthetic paper was made in the following In such cases, particularly inthe case of a polyconmanner. A polypropylene of a MI of 0.8 was heatedand densable monomer, the monomer may be one wherein kneaded in alleXtmdel' Operated at and extruded condensation has progressed to acertain extent. through a die into a Whiel} W38 eqoled to a p Thepolymerizable monomer may be introduced into mm below Y whereupon an themicrovoids of the synthetic-resin film, in general, by t q fi wasobtamed- T1115 film f sfretched a method such as contacting the film andmonomer under m gz figs length by means of a long'mdmal suctch' $1 55 3g gff gjgxgg then applymg thereto A composition was prepared separatelyby mixing 15 percent by weight of talc and 15 percent by weight of Themfmomer thfls mtroFluced the mlfmvolds m titanium white with apolypropylene of a MI of 4.00. the film 1s polymerized by impartingappropriate p y This composition was laminated by extrusion onto oneenzatlon eOndltlons to the monomer- The most Pf surface of the abovedescribed film monoaxially stretched sentative procedure, in the case ofa monomer having i h longitudinal direction. ethylenically unsaturatedbonds. comprises adding before- The resulting laminated sheet was thenheated and hand a radical polymerization initiator to the monomerstretched 7.5 times its original width in the transverse and heating themonomer with initiator. Another method 75 direction and was then cooledin its as-stretched state. The

lateral edges of this sheet were trimmed off, and the sheet was wound.The thicknesses of sheets thus made are indicated in Table 2.

The PE synthetic paper was made in the following manner. A compositionof 60 percent of high-density polyethylene, 20 percent of polypropylene,and 20 percent of a fine filler, all percentages being by weight, washeated and kneaded in an extruder operated at 230 C. and was thenextruded through a die to form a sheet, which was then cooled to atemperature below 40 C., whereupon an unstretched sheet was obtained.

This sheet was biaxially stretched 4 x 4 times its original dimensionssimultaneously at a stretching speed of 180 cm./minute in eachdirection, and the sheet thus obtained was cooled while being maintainedin its as-stretched state. The edges were trimmed oil.

The PE film was a =film of a thickness of 50 microns in whichhigh-density polyethylene of a MI of 0.3 and a specific gravity of 0.95was used.

The Clark stiffnesses were determined in conformance with TAPPI/T-45l.

We claim:

1. A process for producing opaque synthetic-resin films of improvedstifiness which comprises:

(a) preparing a synthetic-resin film containing a fine particulatefiller uniformly blended therewith;

(b) stretching said synthetic-resin film thereby to produce a filmhaving microvoids, dispersed in those regions of the film near the outersurface thereof communicating with the outside of the film and beingsubstantially discontinuous in directions perpendicular to the outersurface of the film;

(c) introducing a polymerizable monomer into said microvoidscommunicating with the outside; and

(d) polymerizing said monomer within said microvoids.

2. A process as claimed in claim 1 in which said synthetic-resin filmcomprises a film of a polymer of an olefin selected from the groupconsisting of ethylene, propylene, and butene-l.

3. A process as claimed in claim 1 in which said polymerizable monomeris an ethylenically unsaturated monomer.

4. A process as claimed in claim 1 in which said monomer is introducedinto the microvoids by impregnating the synthetic-resin film with asolution containing the monomer.

5. A process as claimed 1 in which said monomer is polymerized withinsaid microvoids by irradiation with radiation rays.

References Cited UNITED STATES PATENTS 2,320,536 6/1943 Pollack et a1.117-161 UB UX 3,250,642 5/1966 Parasacco et a1. 117138.8 E 3,552,9861/1971 Bassernir et al 117-9331 3,676,195 7/1972 Kehr et a1. 117-138.8 A

WILLIAM D. MARTIN, Primary Examiner T. G. DAVIS, Assistant Examiner Us.01. X.R.

117-47 A, 93.31, 138.8 B, 138.8 N, 138.8 PV, 138.8 VA

